Avoiding Seal Failure Department Editor: Kate Torzewski Chemical Engineering

Seals are assemblies of elements that prevent the passage of a solid, liquid, gas or vapor from one system to another. When a seal allows leakage of material, failure has occurred. This guide provides an overview of common seal types and a discussion of seal failure to aid in choosing the most effective seal and avoiding future failure.

Seal types
Seals types can be classified within two broad categories: static and dynamic. Static seals have no relative motion between mating surfaces, while dynamic seals do have relative motion between a moving surface and a stationary surface. Seals do not have to fit into one category or the other; rather, seal types can fall anywhere on a spectrum between static and dynamic, and few seals are strictly one type or the other. Table 1 describes the applications and requirements of several common seal types.

Seal failure

Seal failure is caused by a wide variety of circumstances, including improper installation and environmental factors such as temperature, pressure, fluid incompatibilities, time and human factors.

Most causes of failure can be described as mechanical difficulties or system operations problems. Examples of mechanical difficulties include strain on the seal face caused by improper installation and vibration caused by improper net positive suction head. Meanwhile, system operating problems can include conditions that are outside of a pump’s best performance envelope, such as upsets, dry running, and pressure or temperature fluctuations. Changes in the fluid being processed can cause problems as well, especially with fluids that flash or carbonize.

Common visual indicators of failure include short cuts, V-shaped notches in the seal, skinned surface in localized areas, or thin, peeled-away area on the seal.

Table 2 describes causes of some of the most prevalent types of seal failure with recommended methods of action.In some cases, the cause of failure may be difficult to determine due to the complexity of the seal construction. These unique failure modes can result in flaking or peeling of the seal face, corrosion, flaking or pitting of the carbon faces, degradation of the elastomer energizer seals, and spring or bellows breakage. It is likely that these rapid degradations are a result of contamination, which can be avoided with careful installation or using pre-assembled, cartridge-type mechanical seals.

References
1. Ashby, D. M. Diagnosing Common Causes of Sealing Failure, Chem. Eng. June 2005, pp. 41–45.
2. Netzel, J., Volden, D., Crane, J. Suitable Seals Lower the Cost of Ownership, Chem. Eng. December 1998, pp. 92–96.